A suction bulb or bulb syringe is an item made of rubber (or a similar material) that contains a volume of air in an enclosed container connected to an opening. When the bulb is compressed, air is forced out of it through an opening. When the pressure is released, the bulb will suck air back in, or try to do that.
For adult play
Types of suction bulbs (nipple bulbs) can be used to erect a woman's or man's nipples. Similar devices (nipple suckers, nipple suction sets) can work on slightly syringe style mechanisms, and nipple suction cups work on the suction cup principle.
Such devices are also popular in BDSM play, e.g. as part of nipple torture. Other means to temporarily enlarge nipples include oral sucking, manual stimulation (pinching, twisting), rubber bands and nipple clamps.